Before beginning recipe, have mushrooms and water chestnuts chopped and set aside.
Prepare stir fry sauce by adding ingredients to a small bowl and mixing well with a small whisk or fork, set aside.
Make the special sauce by dissolving the sugar in the water in a small bowl. Add the soy sauce, rice wine vinegar, ketchup, lemon juice, and sesame oil. Mix well and set aside until ready to serve. Combine the hot water with the hot mustard and set this aside as well.
Bring 1 tablespoon oil to high heat in a wok or large frying pan. Saute ground chicken until cooked through.
With the pan still on high heat, add the other tablespoon of oil. Add the garlic, onions, water chestnuts, and mushrooms to the pan. Add the stir fry sauce to the pan and saute the mixture for a couple minutes.
To complete the "special sauce," add your desired measurement of the mustard and garlic chili sauce to the special sauce mixture to reach preferred spiciness.
Serve the chicken mixture in the lettuce cups and top with "special sauce" as desired.